The present invention generally relates to an image reading apparatus, and more particularly to an image reading apparatus, including a digital copier, a facsimile and an image file apparatus, which can output an image signal in accordance with a brightness of a specified region of an original document.
Generally, an image reading apparatus uses a line sensor having a number of photoelectric conversion elements (for example, charge coupled devices) for reading an image of each original document. A number of photoelectric conversion elements are aligned in the line sensor along a main scan line to achieve a resolution equivalent to some lines per 1 millimeter or some tens of lines per 1 millimeter for each line of data that the line sensor supplies. To read two-dimensional image information of an original document by the image reading apparatus, a main scan is made by scanning of an optical lighting part in a main scanning direction, and, in conjunction with the main scan, a sub scan is made by transporting the original document by one pitch in relation to the line sensor in a sub scanning direction perpendicular to the main scanning direction. Such an image reading apparatus is disclosed, for example, in Japanese Laid-Open Patent Application No. 62-235872. Conventionally, however, there is a problem in that the conventional image reading apparatus often experiences undesired variations of an output level of an image signal supplied when the image of the same original document is read. In such image reading apparatus, the output level of the image signal supplied when the image of an original document is read varies, due to a change in illuminance of light which is supplied by the optical lighting part to a surface of the original document, even when the same original document is read by the image reading apparatus.
This problem is caused primarily because the image signal actually outputted from the photoelectric conversion elements is a reflectance (or, brightness of a region of an original document) multiplied by the illuminance of light supplied by the optical lighting part. As a measure for eliminating this problem, a reference white board is provided outside an effective image reading region of the line sensor along the main scan line. An output signal as a reference signal is obtained by the line sensor from this reference white board outside the effective image reading region, and this reference signal is used for normalizing a level of an image signal obtained from an image of an original document within the effective image reading region by the line sensor, which is based on the reference signal. The image reading apparatus therefore can supply an image signal indicative of the actual brightness of the image of the original document, even when the illuminance of light supplied from the optical lighting part varies.
There is another cause of the above undesired variations of the output level of an image signal supplied when an original document is scanned. This cause of the undesired variations of the output level of the image signal is the relative brightness of a specified region of the original document relative to a brightness of a reference white board. The reference white board as described above is provided at an end portion of the main scan line which is adjacent to the specified region of an original document in the image reading apparatus for the purpose of eliminating the former problem, but, a level of the reference signal obtained from the reference white board is significantly varied depending on the relative brightness of a specified region of an original document. That is, the level of the reference signal varies depending on what kind of original documents are read, and the level of an image signal outputted is undesirably varied even when the same original document is read. For example, in a case where a small black region of an original document (image data) is surrounded by a large white region within the original document, the area of the large white region being much greater than that of the small black region, a level of an image signal slightly higher than the actual signal level is produced from such a small black region. And, in a case where a small white region within an original document is surrounded by a large black region of the same, a level of the resulting image signal when the original document is read by the image reading apparatus which is slightly darker than the actual signal level is produced with the small white region. For this reason, the level of the reference signal obtained from the reference white board is sometimes varied depending on the level of brightness of a surrounding region of the reference white board. A major cause of this problem is a recurring reflection of light supplied by the optical lighting part to a surface of the original document. This light from the optical lighting part is repeatedly reflected between the surface of the original document and a surface of the optical lighting part. In this manner, the output level of an image signal when the original document is scanned along the main scan line varies due to the variation of the level of the reference signal obtained from the reference white board. In a case where a certain kind of an original document is read by the image reading apparatus, the image reading apparatus does not supply an accurate level of an image signal from each original document, because the level of the reference signal from the reference white board is unsuitably varied depending on the relative brightness of a specified region of each original document relative to the brightness of the reference white board. In the conventional image reading apparatus, it is therefore difficult to always supply an accurate level of an image signal from original documents, regardless of what kind of original documents is inputted to the conventional image reading apparatus.